The present invention concerns lift trucks with telescopic hoisting arm.
Usually these lift trucks have a chassis carrying a front axle and a rear axle, on which chassis the telescopic arm is mounted.
This hoisting arm is often mounted on a fixed bracket disposed in the rear of the chassis, but it has also been proposed, for example by French Patent No. 1,182,493, to mount the arm on the chassis by means of a variable, i.e., movable, quadrilateral linkage. To this effect, connecting means are provided for connecting two points situated in back of the arm to two points of the chassis.
In the form of realization of that patent, such an arrangement permits assuring a vertical rise of the load, but it can be noted that the truck according to that patent does not permit lifting the loads to great heights and that moreover the hoisting arm hinders the operator's vision practically along its entire displacement.
The main disadvantage of the known lift trucks resides in the loss of handling capacity both horizontally and vertically as the telescopic hoisting arm rises.
To illustrate this fact, FIG. 1 shows schematically the diagram of the hoisting capacities of a known lift truck, here a truck whose arm is articulated on a fixed bracket above the rear axle. The superimposed elements at the upper right of FIGS. 1 and 6 represent the fork at the end of the hoisting arm which carries a load normalized at a volume of one cubic meter, i.e. 1000 millimeters on each side. The center of gravity of the load is 500 millimeters from the faces of the fork in both height and width. On the graph the horizontal axis H is the distance between the center of gravity of the load and the front side of the front wheels of the truck and the vertical axis V is the distance between the center of gravity of the load and the ground on which the trunk wheels ride, the ground assumed to be horizontal. The curves within the graph is the weight of the maximum load of the normalized dimensions which can be supported by the lift truck without any rocking.
It is seen that heavy loads, that is of between 2500 and 3000 kg, carried on a deck which not only is small but is situated more than four meters from the ground is unworkable since the truck cannot, without rocking, support the load between the ground and this height.
In order to limit the risks of rocking of the truck, one is obliged to provide the use of stabilizers carried by the chassis of the machine and taking support on the ground. These stabilizers artifically increase the stability of the machine and may lead the operators to take more and more chances in counting on them.